Add prelude module to generated server crate

[hlbarber]

The purpose of a prelude module is to re-export items commonly used in everyday use of the library. Adding it to the generated crate would mean customers (and our own examples) wouldn't have to use pokemon_service_server_sdk::... repeatedly to collect all the base imports.

Proposed items to re-export:

• service::*
• operation_shape::*
• InstrumentExt

[crisidev]

IMHO we should also figure out what to re-export from aws-smithy-http-server so that most of the customers will not have to depend directly on it.

A couple of things that come to mind are the Router and AddExtensionLayer.

[hlbarber]

After some discussion we've decided to re-export the main codegenerated types from the root module and not re-export lesser items such as inputs/outputs.

Additionally, #1506 should be done using a pokemon_service_server_sdk::server module perhaps.